86 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



to that of one atmosphere and if the roots still con- 

 tinue to absorb water in greater quantities than the 

 leaves transpire it (as may actually occur), we have 

 the phenomenon of " weeping" or " bleeding" : similar 

 effects are produced when the sun's rays directly raise 

 the temperature of thin twigs the expanding air- 

 bubbles drive water before them, as had already been 

 shown by Sachs and others. Hartig concludes that the 

 cause of the ascent of the water in a tree is to be 

 found in the differences of pressure (density) of the 

 air-bubbles imprisoned in the tracheides, &c. : the water 

 is driven from lumen to lumen in the direction of least 

 pressure. 



Taking the simple tracheide system of Conifers, the 

 elements are in contact on the one hand with the 

 mesophyll of the leaves, in the venation, and on the 

 other hand with the parenchyma of the roots ; the 

 walls in contact are the thinnest of all, and water easily 

 filters through them, but they need strengthening 

 lattice work theraisvn d'etre for rings, spirals, &c., and 

 it is interesting to note that this kind of support only 

 occurs in the proto-xylem, the part which alone comes 

 into direct contact with the above-named cells. The 

 trunks of trees, &c., would be impossible, however, if 

 the secondary wood were not provided with more 

 support ; hence we find firm, thick-walled organs in it, 



